1. Field of the Invention
The present invention relates to a frequency conversion apparatus used in a tuner portion of a cable television receiver or the like.
2. Description of the Prior Art
FIG. 8 is a block diagram of a conventional configuration of a double-conversion tuner apparatus. The double-conversion tuner apparatus extracts a desired frequency component from the high frequency signal (hereinafter refereed to as an RF (radio frequency) signal) fed thereto, and generates an intermediate-frequency signal (hereinafter referred to as an IF (intermediate frequency) signal) that is easily handled by the detector circuit provided in the succeeding stage.
As shown in FIG. 8, the conventional double-conversion tuner apparatus includes a first band-pass filter 10a, an attenuator 11, a high-frequency amplifier 12, a first local oscillator 13, a first mixer 14, a second band-pass filter 15, a second local oscillator 16, a second mixer 17, a third band-pass filter 18, and an intermediate-frequency amplifier 19.
Now, the frequency conversion operation of the double-conversion tuner apparatus configured as described above will be described with reference to FIG. 9. In the double-conversion tuner apparatus configured as described above, a reception signal that has passed through the first band-pass filter 10a is mixed with a first local oscillation signal (having a frequency flo) in the first mixer 14. Thus, the signal of the reception channel (having a frequency f1) is converted into a first IF signal (having a frequency flo-f1).
On the other hand, if the signal of a channel (having a frequency f2) other than the reception channel passes through the first band-pass filter 10a, the signal of this other channel is converted into an unwanted signal (having a frequency flo-f2) by the first mixer 14. Here, if the frequency f2 of this other channel is higher than the frequency f1 of the reception channel, and the frequency flo-f2 of the unwanted signal is within the pass band of the first band-pass filter 10a, the unwanted signal leaks through the first band-pass filter 10a to the terminal IN. This phenomenon is called back talk, and adversely affects the frequency conversion and other characteristics of the double-conversion tuner apparatus.
For this reason, in conventional double-conversion tuner apparatuses, as a technique of reducing unwanted signals, such as back talk, that leak from inside the apparatus to the terminal IN, configurations as shown in FIGS. 10A to 10C are adopted. In the configuration shown in FIG. 10A, between the high-frequency amplifier 12 and the first mixer 14, there is added an isolation amplifier A of which the input and output are isolated from each other. In the configuration shown in FIG. 10B, the first band-pass filter 10a is replaced with a split-band-type band-pass filter 10b. In the configuration shown in FIG. 10C, the first band-pass filter 10a is replaced with a variable band pass filter 10c of which the cut-off frequency can be controlled.
It is true that, by configuring double-conversion tuner apparatuses in these ways, it is possible to reduce the leakage of unwanted signals, such as back talk, and thereby enhance the frequency conversion and other characteristics of the double-conversion tuner apparatuses.
However, in the double-conversion tuner apparatus shown in FIG. 10A, the addition of the isolation amplifier A disadvantageously increases costs and degrades distortion characteristics. In the double-conversion tuner apparatus shown in FIG. 10B, building the split-band-type band-pass filter requires a plurality of band-pass filters. This disadvantageously increases the number of components, and thus increases costs and trouble in assembly. In the double-conversion tuner apparatus shown in FIG. 10C, reduction of back talk can be realized at lower cost than in the above two configurations, but suffers from variation of the characteristics of the band-pass filter 10c with the reception frequency. This disadvantageously causes variation of input return loss characteristics (see FIGS. 11A and 11B).